CVD-coated cemented carbide insert for toughness demanding short hole drilling operations

Method of making coated cemented carbide inserts with excellent toughness properties comprising providing a cemented carbide substrate WC having an average grain size of from about 0.5 to about 1.5 μm, from about 8 to about 11 wt-% Co, and from about 0.2 to about 0.5 wt-% Cr by wetmilling powders with pressing agent, and small additions of carbon black or pure tungsten powder to obtain a CW-ratio in the sintered inserts of from about 0.80 to about 0.90 in a slurry, drying the slurry to a powder, compacting and sintering and after conventional post sintering treatment depositing a coating comprising:

• a first, innermost layer of TiCxNyOz with x+y+z=1, with equiaxed grains with size less than about 0.5 μm and a total thickness less than about 1.5 μm, using known CVD-methods,
• a layer of TiCxNyOz with x+y+z=1 with a thickness of from about 1 to about 8 μm, with columnar grains and with an average diameter of about less than about 5 μm by MTCVD-technique with acetonitrile as the carbon and nitrogen source for forming the layer in the temperature range of from about 700 to about 900° C.,
• a layer of smooth fine-grained grain size from about 0.5 to about 2 μm Al2O3 consisting essentially of the κ-phase with a thickness of from about 0.5 to about 5 μm, using known CVD-methods,
• a further layer less than about 1 μm thick, of TiCxNyOz with x+y+z=1 using known CVD-methods,
• fully or partly removing the outermost TiCxNyOz-layer and Al2O3-layer on at least 80% of the rake face surface area between the edge line and 100 μm inwards in the direction perpendicular to the edge line.